1. Field of the Invention
The present invention relates to a method for controlling the degree of side-etch occuring in case of selectively etching a solid oxide thin film, especialy of silicon oxides, which is used for a semiconductor device and which has surface hydroxyl groups on its surface.
2. Description of the Prior Art
Photo-etching is a technique for protecting selected areas of the surface of a thin film by an organic polymer and etching the remaining portion of the surface in the manufacture of various articles such as semiconductor devices and printed circuit cards.
Concretely, the photosensitive organic polymer generally termed "photoresist" (including two sorts of the negative type photo-resist and the positive type photoresist, the former of which is polymerized and becomes insoluble in a solvent when exposed to light and the latter is depolymerized and becomes soluble when exposed to light) is applied onto the surface of the thin film. The photoresist is exposed to light through a mask of black and white, to polymerize the photoresist at the selected areas (depolymerize the same in case of the positive type photoresist) and render it insoluble (soluble in case of the positive type photoresist) in the solvent. Thereafter, the photoresist in the unpolymerized portion is dissolved and removed. Through a photoresist mask thus formed on the thin film, the selected areas of the thin film are chemically etched.
The prior art photo-etching technique has the disadvantage that when the film material with the photoresist applied as the mask is immersed in an etchant, the etchant permeates between the photoresist material and the thin film material due to inferior affinity between both the materials. This gives rise to the side-etch (the proceeding of etching to the part of the thin film to-be-protected), so that a fine pattern is not etched at good reproducibility.
In some extreme cases, the photoresist exfoliates from the surface of the thin film, and its effect as the mask is fully lost.
It is considered that such phenomena are prone to arise where the affinity between the thin film and the photoresist is bad. In order to reduce the side-etch, a variety of measures have hitherto been adopted. These measures include conducting the baking after applying the photoresist on the thin film material, adding an adhesion accelerator or a surface-active agent at the etchant and changing the composition or temperature of the etchant so as to increase the etching rate.
Since, however, the property of the surface of the thin film greatly differs in dependence on, not only the sort of the material of the thin film, but also the process history, etc. thereof, it has been very difficult with the prior-art technique to perform a photo-etching working of high finishing accuracy at good reproducibility.
In the case of subjecting a thin film material, 1 .mu.m - several .mu.m thick, to the photo-etching working, a phenomenon just converse to the foregoing problem is sometimes the cause for generating defectives. For example, where after forming an uneven pattern in the first thin film of a thickness of several .mu.m by the photo-etching working, the second thin film is to be formed thereon, it can occur that the formation of the second thin film is partially obstructed by the unevenness of the first thin film or that the second thin film formed severs on the end part of a convex portion of the first thin film.
In such case, it is desirable to make the unevenness of the first thin film less severe. In other words, it is desirable that the degree of side-etch can be controlled in the photo-etching working.